I don't think the past should be private. Especially a persons criminal history. You want to keep something private, how about your sexual orientation and activities.
@geronl, I should have answered this days ago.
I do think one's past should be private, with the exception of adult criminal history, and things deemed "public information", such as real properties owned or legal actions like divorces and bankruptcies. Why should anyone have a right to know anything about you or me that we don't chose to share with them?